In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having the same frame length with the same number of subframes. The subframes are configures to perform uplink (UL) transmission or downlink (DL) reception in different Duplexing methods. Time-division duplex (TDD) is the application of time-division multiplexing to separate transmitting and receiving radio signals. TDD has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. Seven different TDD configurations are provided in LTE/LTE-A to support different DL/UL traffic ratios for different frequency bands. Specifically, each radio frame contains ten subframes, and each subframe is defined to be an uplink subframe or a downlink subframe for each of the TDD configuration.
In 3GPP LTE/LTE-A systems, one of the promising technologies to enhance the data throughput is carrier aggregation (CA), where multiple component carriers (CCs) are aggregated and jointly used for transmission to/from a single device. In contiguous carrier aggregation, two or more contiguous CCs in a frequency band are aggregated. In non-contiguous carrier aggregation, two or more non-contiguous CCs are aggregated. For non-contiguous CA, there are intra-band CA and inter-band CA. In intra-band carrier aggregation, two or more non-contiguous CCs in the same frequency band are aggregated. In inter-band carrier aggregation, two or more non-contiguous CCs in different frequency bands are aggregated.
In Rel-11 CA enhancement work item (RP-110451), it is agreed to support inter-band CA such that multiple CCs in different bands having different TDD configurations can be aggregated. The objective of inter-band CA is to support flexible aggregation, to enhance DL data throughout, and to improve UL transmit power efficiency. Accordingly, potential issues on carrier aggregation with different TDD configurations are investigated and associated solutions are proposed.